1. Field of the Invention
The invention is concerned with methods for the manufacture of highly filled composite materials consisting of finely powdered filler material in a matrix of polymer material, and new highly filled composite materials made by such methods.
2. General Background and State of the Art
The electronics industry is an example of one which makes substantial use of substrates as supports and dielectric participants for electronic circuits, such substrates consisting of thin flat pieces produced to exacting specifications as to starting material and physical and electrical properties. The history of the industry shows the use of progressively higher operating frequencies and currently for frequencies up to about 800 megahertz (MHz) copper coated circuit boards of glass fiber reinforced polymers, such as epoxies, cyanide esters, polytetrafluoroethylene (PTFE) and polyimides, have been and are still used. One popular laminate material for such applications is known as FR-4, consisting of epoxy resin deposited on a woven glass fabric, because of its ease of manufacture and low cost. Typically this material has a dielectric constant of 4.3-4.6 and a dissipation factor of 0.016-0.022 and is frequently used in computer related applications employing frequencies below about 500 MHz. The lowest possible value of dielectric constant is preferred in computer applications to improve signal speed. Some computers now operate at 2.0 GHz, while mobile telephones now operate at frequencies of 1-40 GHz, with the prospect of higher frequencies in the future. At higher operating frequencies above approximately 0.8 GHz, FR-4 and similar materials, despite their low cost, are no longer entirely suitable, primarily because of unacceptable dielectric losses, excessive heating, lack of sufficient uniformity, unacceptable anisotropy, unacceptable mismatch of thermal expansion between the dielectric material and its metallization, and anisotropic thermal expansion problems as the operating temperatures of the substrates fluctuate. These thermal expansion problems result from the relatively large coefficients of thermal expansion of the polymers used as substrate material, and the unequal expansion coefficients of reinforcing fibers in their length and thickness dimensions. For frequencies above 800 MHz the dielectric material of the substrates becomes an active capacitive participant in signal propagation and here the current materials of choice are certain ceramics formed by sintering or firing suitable powdered inorganic materials, such as fused silica; alumina; aluminum nitride; boron nitride; barium titanate; barium titanate complexes such as Ba(Mg⅓Ti⅔)O2, Ba(Zr,Sn)TiO4, and BaTiO3 doped with Sc2O3 and rare earth oxides; alkoxide-derived SrZrO3 and SrTiO3; and pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also been employed consisting of metal and semiconductor powders embedded in a glass or polymer matrix, a particular preferred family of polymers being those based on PTFE.
For example, ceramic substrates that have been used for hybrid electronic circuit applications comprise square plates of 5 cm (2 ins) side, their production usually involving the preparation of a slurry of the finely powdered materials dispersed in a liquid vehicle, dewatering the slip to a stiff leathery mixture, making a xe2x80x9cgreenxe2x80x9d preform from the mixture, and then sintering the preform to become the final substrate plate. The substrates are required to have highly uniform values of thickness, grain size, grain structure, density, surface flatness and surface finish, with the purpose of obtaining substantially uniform dielectric, thermal and chemical properties, and also to permit the substantially uniform application to the surfaces of fine lines of conductive or resistive metals or inks.
Such sintered products inherently contain a number of special and very characteristic types of flaws. A first consists of fine holes created by the entrainment of bubbles in the ceramic pre-casting slip of sizes in the range about 1-20 micrometers; these bubbles cannot be removed from the slip by known methods and cause residual porosity in the body. As an example, sintered alumina substrates may have as many as 800 residual bubble holes per sq/cm of surface (5,000 per sq/in). Another flaw is triple-point holes at the junctions of the ceramic granules when the substrate has been formed by roll-compacting of spray-dried powder; they are of similar size to the bubble holes and appear in similar numbers per sq/cm. Yet another consists of xe2x80x9cknit-linesxe2x80x9d, which are webs or networks of seam lines of lower density formed at the contact areas between butting particles during cold pressing. Two other common flaws are caused by inclusions of foreign matter into the material during processing, and the enlarged grains caused by agglomeration of the particles despite their initial fine grinding. The usual inclusions are fine particles due to abrasive wear of the grinding media in the mills. Both inclusions and agglomerates will sinter in a matrix at a different rate from the remainder of the matrix and can result in flaws of much greater magnitude than the original inclusion or agglomerate.
Costly mirror-finishing by diamond machining and lapping of the ceramic surfaces is required to allow the accurate deposition of sputtered metallization layers from which conductor lines are formed by etching. Mirror-finishes are required because the electrical currents at frequencies above 0.8 GHz move predominantly in the skin of a conductor line, while in the lower frequencies they occupy the entire cross-section. The thickness of the skin through which currents move at GHz frequencies becomes thinner as frequencies rise and are already as thin as 1 to 2 micrometers in copper at around 2 GHz. Any surface roughness of the conductors will therefore contribute to conductive losses. For example, at a frequency of 4 GHz, the conductive loss at of the interface between conductor and substrate is 1.65 times higher with an RMS roughness of 40, compared to an RMS roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids, R. Brown, published 1989 by the International Society for Hybrid Microelectronics of Reston, Va.)
There is therefore continuing interest in methods for manufacturing composite materials for the production of electronic substrates and for use as electronic packaging materials, with which sintering and the high processing temperatures required together with their attendant difficulties, high cost of diamond machining and lapping, and the associated considerable costs are avoided.
The low inherent mechanical strength of the currently available matrix forming polymers and their excessive thermal expansion coefficients has made it necessary to embed reinforcing materials, such as woven glass fiber cloth, into the substrate body, to strengthen it and also to constrain this excessive thermal expansion. But such reinforcing materials unfortunately cause unacceptable inhomogeneity of the structure. For example, their presence makes it difficult to incorporate powdered filler materials into the body of the substrate with a high degree of uniformity. Another difficulty is caused by the generally poor adhesion exhibited by the commercially available matrix polymers toward the usual filler materials, and extensive research and development has been undertaken in the past, and is continuing, in connection with known substrate-forming polymers, such as PTFE, to find coupling agents that will provide adequate adhesion between the polymer and the powder components, and thus satisfactory mechanical strength in the resultant substrates.
Dielectric materials are commonly used as insulating layers between circuits, and layers of circuits in multilayer integrated circuits, the most commonly used of which is silica, which in its various modifications has dielectric constants of the order of 3.0-5.0, more usually 4.0-4.5. Low values of dielectric constant are preferred and organic polymers inherently usually have low dielectric values in the range 1.9-3.5, so that considerable research and work has been done to try to develop polymers suitable for this special purpose, among which are polyimides (frequently fluorinated), PTFE, and fluorinated polyarylene ethers, some of the materials having dielectric constants as low as that of air, i.e. 1.00.
U.S. Pat. Nos. 5,658,994, issued Aug. 19, 1997, and 5,874,516, issued Feb. 23, 1999, both to Air Products and Chemicals, Inc. of Allentown, Pa, the disclosures of which are incorporated herein by this reference, disclose and claim a unique utility as a dielectric coating material for micro-electronic devices of a class of polyarylene ethers as a replacement for silica-based dielectric materials, wherein the polyarylene ether does not have non-aromatic carbons (other than perphenylated carbon), fluorinated substituents or significantly polarizable functional groups. These materials, which are relatively easily synthesized, are found surprisingly to have an excellent combination of desirable properties, namely thermal stability, low dielectric constant values, low moisture absorption and low moisture out gassing.
U.S. Pat. No. 5,658,994 discloses and claims in its broadest aspect an article of manufacture comprising a combination of a dielectric material and a microelectronic device, wherein the dielectric material is provided on the microelectronic device and contains a polyarylene ether polymer consisting essentially of non-functional repeating units of the structure:
xe2x80x94{xe2x80x94Oxe2x80x94Ar1xe2x80x94Oxe2x80x94Ar2xe2x80x94}mxe2x80x94{xe2x80x94Oxe2x80x94Ar3xe2x80x94Oxe2x80x94Ar4xe2x80x94}nxe2x80x94
wherein m=0 to 1.0; and n=1.0-m; and Ar1, Ar2, Ar3 and Ar4 are individually divalent arylene radicals selected from the group consisting of: phenylene; biphenyl diradical; para-terphenyl diradical; meta-terphenyl diradical; ortho-terphenyl diradical; naphthalene diradical; anthracene diradical; phenanthrene diradical; diradicals of 9,9-diphenylfluorene of specific type; and 4,4xe2x80x2-diradical of dibenzofuran and mixtures thereof, but Ar1, Ar2, Ar3, and Ar4, other than the diradical 9,9-diphenylfluorene, are not isomeric equivalents.
U.S. Pat. No. 5,874,516 claims polyarylene ethers consisting essentially of non-functional repeating units of the structure:
xe2x80x94{xe2x80x94Oxe2x80x94Arxxe2x80x94Oxe2x80x94Ar1xe2x80x94}mxe2x80x94{xe2x80x94Oxe2x80x94Ar2xe2x80x94Oxe2x80x94Ar3xe2x80x94}nxe2x80x94
wherein m=0.2 to 1.0; and n=1.0-m; and Ar1, Ar2, and Ar3 are individually divalent radicals selected from the group defined in the preceding paragraph; or essentially of non-functional repeating units of the structure:
xe2x80x94{xe2x80x94Oxe2x80x94Arxxe2x80x94Oxe2x80x94Ar1xe2x80x94}mxe2x80x94{xe2x80x94Oxe2x80x94ArxOxe2x80x94Ar3xe2x80x94}nxe2x80x94
wherein m=0 to 1.0; and n=1.0-m; Arx is a special radical 9,9-bis(4-oxyphenyl)fluorene and Ar1 and Ar3 are individually divalent radicals also selected from the group defined in the immediately preceding paragraph. Variations in Ar1, Ar2, Ar3 and Ar4 are stated to allow access to a variety of properties such as reduction or elimination of crystallinity, modulus, tensile strength, high glass transition temperature, etc. The polymers are said to be essentially chemically inert, have low polarity, have no additional functional or reactive groups, and to be thermally stable at temperatures of 400xc2x0-450xc2x0 C. in inert atmospheres.
The specified polymers are non-functional in that they are chemically inert and they do not bear any functional groups that are detrimental to their application in the fabrication of microelectronic devices. They do not have carbonyl moieties such as amide, imide and ketone, which promote adsorption of water. They do not bear halogens such as fluorine, chlorine, bromine and iodine, which can react with metal sources in metal deposition processes. They are composed essentially of aromatic carbons, except for the bridging carbon in the 9,9-fluorenylidene group, which has much of the character of aromatic carbons due to its proximity to aromatic structures; for the purposes of the invention the carbon is deemed to be a perphenylated carbon.
The polymers are proposed for use as coatings, layers, encapsulants, barrier regions or barrier layers or substrates in microelectronic devices. These devices may include, but are not limited to multichip modules, integrated circuits, conductive layers in integrated circuits, conductors in circuit patterns of an integrated circuit, circuit boards as well as similar or analogous electronic structures requiring insulating or dielectric regions or layers. They are also proposed for use as a substrate (dielectric material) in circuit boards or printed wiring boards. Such a circuit board has mounted on its surface patterns for various electrical conductor circuits, and may include various reinforcements, such as woven nonconducting fibers, such as glass cloth. Such circuit boards may be single-sided, double-sided or multilayered.
Although the above discussion of prior art proposals for filled polymer substrates has referred almost entirely to the support substrates used in the electronics industry there are many other products in which such filled materials are used, and in which the highest possible filling with the filler material is of advantage. Examples are magnets, ferrite antennae, resistors and capacitors.
The principal object of the invention is to provide new methods for manufacturing highly-filled composite materials consisting of particles of finely powdered filler material bonded together in a matrix of polymer material, such new composite materials, and articles made from such composite materials.
It is another object to provide such new methods with which the resultant composite materials and articles comprise at least 60 percent by volume of filler material, with the remainder consisting of the polymer material matrix together with the residue of any additives employed in their production.
It is a further object to provide such new methods which are operable to produce composite materials and articles comprising at least 60 percent by volume of the filler material, with the remainder consisting essentially of the polymer material matrix, employing as the polymer material polymers soluble in a volatilisable solvent and adequately adhesive to the filler material to provide a minimum flexural strength in the resultant composites.
In accordance with the invention there are provided methods for the production of highly filled composites of finely powdered filler material particles in a polymer matrix comprising the steps of:
forming a solution of polymer in volatilizable solvent, the polymer being of sufficient strength and sufficiently adhesive to the filler material particles to result in composites of flexural strength not less than 17 Mpa (2,500 psi);
mixing together from 60 to 97 by volume percent of filler material particles with sufficient solution to form a suspension having therein the balance in volume percent of the polymer required for the composite;
evaporating solvent from the suspension while subjecting it to high shear treatment so as to maintain distribution of filler particles in the solution with a high degree of uniformity, the evaporation being continued until a mixture is obtained consisting essentially of filler particles with the residual solution distributed with a high degree of uniformity therein, the evaporation being further continued until the solvent has substantially entirely been removed; and
subjecting the mixture to a temperature sufficient to melt the polymer and to a pressure sufficient to maintain the melted polymer dispersed in the interstices between the filler material particles with a high degree of uniformity.
Also in accordance with the invention there are provided highly filled composite materials comprising finely powdered filler material particles distributed in a matrix of polymer material with a high degree of uniformity, the materials comprising;
a composite mixture of 60 to 97 by volume percent of filler material particles and the balance polymer, consisting of a polymer that is soluble in a volatizable solvent that has been volatilized from the mixture, the polymer having sufficient strength and being sufficiently adhesive to the filler material particles to result in composites of bending strength not less than 17 Mpa (2,500 psi);
wherein the composite mixture has been subjected to a temperature sufficient to melt the polymer and to a pressure sufficient to disperse the melted polymer into the interstices between the filler material particles to a high degree of uniformity.